This invention relates to a thin integral cooling and pressure relieving sputtering cathode target assembly for a sputtering apparatus.
One of the most important commercial processes for depositing thin films of a desired material onto a substrate is sputter deposition, also known as sputter coating or sputtering. Sputter deposition is used extensively in many industries including the microelectronics, data storage and display industries to name but a few.
Generally, the term sputtering refers to an xe2x80x9catomisticxe2x80x9d process in which neutral, or charged, particles (atoms or molecules) are ejected from the surface of a target material through bombardment with energetic particles. A portion of the sputtered particles condenses onto a substrate to form a thin film. The science and technology of sputtering is well known and described for example in Vossen, J. L., Kern, W., Thin Film Processes, Academic Press (1978). Sputtering can be achieved through several techniques. Generally, in xe2x80x9ccathodicxe2x80x9d (xe2x80x9cdiodexe2x80x9d) sputtering the target is at a high negative potential relative to other components, usually through application of a negative bias from a power supply, in a vacuum chamber system, typically containing an inert gas or mixture of gases at low pressure. A plasma containing ionized gas particles is established close to the target surface and ionized gas particles are accelerated by the action of the electric field towards the target surface. The bombarding particles lose kinetic energy through momentum exchange processes with the target atoms, some of the latter particles gain sufficient xe2x80x9creversexe2x80x9d momentum to escape the body of the target, to become sputtered target particles. Note a sputtered particle may be an atom, atom cluster or molecule either in an electrically neutral or charged state. A flux of sputtered particles may contain any one or any mixture of such entities.
Coating high aspect ratio structures is of critical importance, e.g., in emerging submicron semiconductor interconnect metalization and high density data storage media applications. In such cases the bounds of application of magnetron sputter deposition is approaching its limit. For example, in coating via type structures in microelectronics interconnect applications, it is well known that sputter deposition suffers from film buildout at the upper edges of the via resulting in a trapped void, xe2x80x9ckeyholexe2x80x9d type film defect as well as other film defects. See, for example, Rossnagel, S., J.Vac. Sci. Tech.B., Vol.16, No.5, p. 2585 (1998). This effect is exasperated with reducing dimensionality and increased aspect ratio. Proponents of current commercial PVD processes assert they can conformally cover relatively high aspect ratio features, or fill relatively high aspect ratio channels or vias, having a critical dimension of at least 0.18 micron, or perhaps greater than 0.13 micron.
Several sputter PVD techniques, many of them developed commercially relatively recently, attempt to control the directionality of the incident sputtered particle flux at a substrate e.g., physical collimation techniques, hollow cathode sputtering, arc sputtering, self ionized sputtering, ionized physical vapor deposition (IPVD) and long throw methods. The latter two techniques probably represent state of the art commercial technologies. The scope, scalability, efficiency and cost considerations of directional sputter technologies have been reviewed by Rossnagel, S., J.Vac. Sci. Tech.B., Vol.16, No.5, p. 2585 (1998). The best techniques utilize tooling and/or process attributes to achieve a degree of control over the angular distribution of incident sputtered particles. These methods are in fact expressly designed to overcome what are believed to be inherent deficiencies in basic magnetron cathode sputter deposition characteristics and target materials design which limit control of the substrate incident angular sputtered flux distribution.
U.S. Pat. Nos. 5,948,215; 5,178,739; and Patent Cooperation Treaty published application No. WO 98/48444 disclose ionized plasma vapor deposition processes, and are incorporated herein by reference.
Long throw methods utilize ballistic (i.e., collisionless) transport and a long throw path to the substrate to xe2x80x9copticallyxe2x80x9d filter the magnetron cathode emitted flux such that only relatively low angle components of the emitted flux (i.e., those close to the target normal) are incident at the substrate. The long throw process is clearly inefficient through flux dilution and suffers from inherent asymmetries in the incident flux. See Rossnagel, S., J. Vac. Sci. Tech. B., Vol.16, No.5, p. 2585 (1998).
Planar Magnetron Sputtering apparatuses are well known Physical Vapor Deposition (PVD) tools commonly used in, for example, the semiconductor industry for the deposition of thin films of metals such as aluminum and its alloys, refractory metals, and ceramics onto a substrate; for example, a silicon wafer or glass sheet being processed. In general, the process of Planar Magnetron Sputtering involves creating and confining a plasma of ionized inert gas over the consumable surface of an energized Cathode Assembly in order to dislodge, by momentum transfer, atoms or molecules from the consumable surface. The consumable surface of a cathode assembly consists of the material to be sputtered and is commonly called a sputter xe2x80x9ctargetxe2x80x9d in Industry. The target is placed a relatively short distance from the substrate in order to improve collection of the ejected target atoms onto the substrate.
Initially, a discharge caused by primarily electrons emitted from the surface of the target, produced by gas ion bombardment of the target resulting from ionization of the gas by natural background ionizing radiation, strike or ignite the plasma. The target is energized by an applied electric field (DC, RF, or both) in an evacuated chamber that is backfilled with an inert gas to typically the 10xe2x88x924-10xe2x88x921 millitorr pressure range. Then, both electrons emitted by the target surface and electrons created by ionizing impacts with the inert gas will be confined near the target surface by means of the magnetron""s magnetic field; which is applied crosswise to the electric field. The created ions accelerate towards the surface of the target and dislodge atoms or molecules from it; many of these atoms or molecules will be directed towards the substrate creating a thin film onto the substrate.
It is well known, that maximum erosion of a target occurs where lines of magnetic flux are parallel to the consumable surface of the target. To increase the sputter-deposition rate for a given applied electric field to the cathode, the magnetron should also ride as close as possible to the side opposite to the consumable surface of the target such that the intensity of the parallel component of magnetic field lines above the target is maximized. Therefore, a design goal is to design a target assembly with as thin a cross-section as possible.
In addition, film-properties, for example uniformity on the substrate, depend greatly on the uniformity of erosion of the target; therefore, other design goals are to design a magnetron that can produce a uniform plasma intensity, and design a drive mechanism that can sweep the magnetron uniformly over the entire target surface.
As the magnetron is swept over the target, considerable energy is dissipated in the form of heat by the ions striking the surface of the target; therefore, the target must be cooled in order to avoid melting the cathode assembly or damaging the equipment. The target in the cathode assembly is normally mounted over a backing plate and cooling means provided to it.
The target assembly in a magnetron sputtering device is generally placed over a sputtering opening of a process chamber to seal the process chamber such that it can be evacuated and then maintained at the low pressures required for the sputter process. The large forces acting on the target assembly due to the pressure differential between ambient atmospheric pressure and the vacuum inside the process chamber require that the target assembly, in particular the backing plate, be designed of considerable thickness to overcome the resultant bending forces. Earlier prior art cathodes were also designed to overcome the bending forces produced by coolants impinging onto the backing plate.
As demand for processing larger substrates continues, and the size of the cathode assembly is scaled accordingly, prior art solved the target assembly bending problems associated with coolants impinging on one side of the backing plate by fitting the target assembly with internal cooling channels.
More recently, prior art has solved the target assembly pressure differential problems associated with applying vacuum from only the process chamber side by evacuating the magnetron housing enclosure which is normally mounted over the target assembly.
FIG. 1 is a simple representation of a sputtering device, shown in U.S. Pat. No. 5,433,835, showing a processing chamber 1 that encloses a substrate 5 to be sputter coated. The substrate 5 is surrounded by a dark space or deposition shield 4 to prevent deposition of material from beyond the edge of the target. A lower insulating ring 2 rests on the top flange of the processing chamber 1. A laminated target assembly 8 is located on the lower insulating ring 2. Inlet and outlet cooling lines 3,9 provide coolant to the internal cooling channels of target assembly 8. An upper insulating ring 7 insulates the top chamber 6 from the target assembly 8. Top chamber 6 is evacuated to equalize the vacuum forces exerted on target assembly 8 by processing chamber 1.
The above mentioned reference, and related U.S. Pat. Nos. 5,487,822 (Jan. 30, 1996), 5,565,071 (Oct. 15, 1996), 5,595,337 (Jan. 21, 1997), 5,603,816 (Feb. 18, 1997), 5,676,803 (Oct. 14, 1997), 5,799,860 (Sep. 1, 1998) solved cooling and pressure differential problems by providing internal cooling channels to the target assembly 8 and fitting a top vacuum chamber 6 over it to essentially equalize the forces imparted by the process vacuum chamber 1; however, the techniques employed introduced several disadvantages.
Main disadvantages are:
The top chamber 6 has to be designed rigid enough to overcome the vacuum forces; therefore introducing design complexity and cost.
The magnetron drive (not shown in FIG. 1) resides inside the top chamber 6; therefore, several vacuum seals must be provided to couple the drive components inside the top chamber 6 to the external components that energize them.
Any maintenance or service to the magnetron-drive mechanism, enclosed by top chamber 6, requires that the whole system be brought to atmosphere in order to avoid bending the now thinly designed target assembly; resulting in increased downtime for the tool.
In order to reduce costs, the same pump is generally used to evacuate or rough both the process vacuum chamber 1 and the top chamber 6. In such case, lubrication to the magnetron drive inside the top chamber 6 needs to be vacuum compatible in order to avoid contaminating the rough pump and high-vacuum components. Vacuum-compatible lubricants are generally more expensive and their lubricating properties normally inferior to standard lubricants.
FIG. 2 shows a detail cross-sectional view of an embodiment of U.S. Pat. No. 5,876,573, a more recent prior art design-variation that also addresses the problems of cooling and pressure differential associated with large target assemblies that seal to the opening of a process chamber; however, at the expense of yet considerable design complexity, as will be explained shortly.
According to this embodiment, a magnetron sputtering system 10 is used to perform sputtering of a target material from a target onto a substrate. The target 16 is mounted to a backing plate 18 that includes several internal cooling conduits 19, the assembly is positioned within a vacuum (processing) chamber 11 defined by chamber walls 39, and held in place by retainer ring 13 which is coupled to bearing support 36. A coolant manifold 12 connects to cooling conduits 19 on the backing plate 18, and to several conduit tubes 40 attached to coolant manifold 12. The conduit tubes 40; which also energize the backing plate 18 and target 16, extend through magnetron assembly housing 21, third insulator ring 35, and bearing support 36 as shown in FIG. 2.
Insulating jacket 34 electrically insulates conduit 40 from magnetron assembly housing 21. A magnet array 15 is positioned above backing plate 18, and enclosed by magnetron assembly housing 21, which provides a housing for the entire magnetron assembly 24, and sits on top of chamber 11.
The magnetron assembly housing 21 is formed to enclose the magnet array 15 and form a space, on the magnet array chamber 22, within the magnetron assembly housing 21. Magnet array chamber 22 comprises a space within magnetron assembly 24 that lies above backing plate 18. In operation, the pressure within the magnet array chamber 22 can be reduced to a pressure much lower than atmospheric by operating a pump through pump port 20 that connects to magnet array chamber 22.
U.S. Pat. No. 5,876,573 seems to retain all of the above-mentioned disadvantages of U.S. Pat. No. 5,433,835; that is:
The magnetron assembly housing 21 has to be designed to withstand the vacuum forces,
Driving the magnet array 22 requires vacuum seals to couple to the motor 38, e.g. ferrofluidic feed-thru 27, etc.
Any service to components enclosed by magnetron assembly housing 21 require that the whole system be brought to atmosphere in order to avoid bending the now thinly designed backing plate 18.
Lubricants used (to extend the life of the bearing) with the KAYDON (Kaydon Corp., Muskegon, Mich.) bearing enclosed by magnetron assembly housing 21 should be vacuum compatible if the pump port 20 is in communication with the (processing) chamber 11; else, an additional pump should be dedicated to evacuate the magnet array chamber 22.
Energized conduit tubes 40 extending through magnetron assembly housing 21, third insulator ring 35, bearing support 36, and mating with coolant manifold 12 need to be kept at atmosphere, as shown by the enclosing sealing devices. These additional sealing devices are required to avoid arcing or glow discharging from the energized conduit tubes 40; which would be the case if conduit tubes 40 were to be exposed to the evacuated magnet array chamber 22.
Additionally, as it has been shown without electrical insulation in U.S. Pat. No. 5,876,573, the exposed to vacuum surfaces of a once energized backing plate 18 within magnet array chamber 22 would very likely either arc to the magnet array 15 or produce a parasitic glow discharge in the evacuated magnet array chamber 22.
The disadvantages of the existing sputtering target systems as described above continue to inhibit the wide use of sputtering as an efficient and cost-effective means for applying surface coatings, particularly to large area substrates.
This invention relates to an improved target assembly for a high productivity sputtering device including a thin integral cooling and pressure relieving structure. This overcomes many of the drawbacks of the previous configurations and provides a structure and method to improve sputtering coverage of large-area substrates.
In particular, the present invention provides a sputtering target assembly comprising:
a sputtering target and target backing plate assembly having opposed first and second sides, the first side providing material for sputtering,
a pressure relief plate having opposed first and second sides, the target and target backing plate assembly second side being in contact with the first side of the pressure relief plate;
heat exchange passages selected from at least one member of the group consisting of:
heat exchange passages defined between opposed sides of the sputtering target and backing plate assembly or defined between opposed sides of the pressure relief plate, and
heat exchange passages defined by heat exchange cavities formed in at least one member of the group consisting of the first side of the pressure relief plate and the second side of the target and target backing plate assembly, wherein the heat exchange passages are formed between the first side of the pressure relief plate and the second side of the target and target backing plate assembly which enclose the heat exchange cavities,
the heat exchange passages having one or more inlet and outlet openings;
an insulation cover unit having opposed first and second sides;
wherein the second side of the pressure relief plate is in contact with the first side of the insulation cover unit to form a vacuum pressure space therebetween capable of maintaining a vacuum therein and the vacuum pressure space has one or more vacuum ports.
Typically, the heat exchange passages are defined by having heat exchange cavities formed in the first side of the pressure relief plate such that, when the first side of the pressure relief plate is contacted to the second side of the target and target backing plate assembly, the heat exchange passages are formed between the heat exchange cavities in the pressure relief plate and the target and target backing plate assembly enclosing those heat exchange cavities, and/or the heat exchange passages are defined by having heat exchange cavities formed in the second side of the target and target backing plate assembly such that, when the first side of the pressure relief plate is contacted to the second side of the target and target backing plate assembly, the heat exchange passages are formed between the heat exchange cavities in the target and target backing plate assembly and the pressure relief plate enclosing those heat exchange cavities.
There is contact of opposed sides of the target and target backing plate assembly and the pressure relief plate in excess of contact of peripheral areas of the target and target backing plate assembly and the pressure relief plate. There is also contact of opposed sides of the pressure relief plate assembly and the insulation cover in excess of contact of peripheral areas of the pressure relief plate assembly and the insulation cover. This contact in the non-peripheral (e.g., central) portions of these opposed sides bordered within the respective peripheral areas can be achieved by these sides having fins/walls extending from the respective sides to contact opposed fin/wall surface or other surface area of the respective opposed side. These contacts in these non-peripheral portions help the target assembly to withstand pressure forces upon or within it.
For example, one embodiment of a sputtering apparatus employing the target assembly of the present invention primarily comprises a sputtering process chamber, a sputtering target assembly, an adjustable magnetron assembly, and provides the sputtering target assembly with integral heating/cooling and pressure relieving passages. A series of cavities/grooves and fins/walls are constructed on both sides of a heat exchanger/pressure relieving plate within the sputtering target assembly. A first side of the heat exchanger/pressure relieving plate is attached to a target backing plate to form heating/cooling passages within the sputtering target assembly. A second or opposing side of the heat exchanger/pressure relieving plate is attached to an insulation cover to form, within the sputtering target assembly, pressure relieving passages with the sputtering process chamber. The target assembly completely covers and seals against a high-vacuum-compatible insulator resting over and sealed to the top flange of the sputtering processing chamber. A magnetron assembly, e.g., a planar magnetron assembly, resting over the target assembly, is independent from vacuum, or vacuum components, and provides means to move or scan about a magnetron or magnet array over the target assembly.
The distance between the magnetron and the target assembly can be adjusted throughout the useful life of the target independent from vacuum, or vacuum components. Depending upon the use, it may be desirable for the magnetron or magnet array to produce an intense, narrow plasma field for an improved target erosion pattern. However, such intense, narrow plasma fields are not necessary, and depending upon the use might not be desired. An insulating sleeve that also centers the target assembly to the sputtering process chamber protects the perimeter of the energized target assembly. In this way large substrates can be sputtered effectively and uniformly without adverse sputtering effects due to target deflection and cooling deficiencies, and without affecting the vacuum integrity in the sputtering process chamber due to service requirements to the magnetron assembly.
These sputtering target assemblies may be employed in magnetron sputtering generally. In some instances they may assist to achieve directional emission characteristics which can be maintained through ballistic transport of the emitted particles and simple geometric considerations, which promote a high degree of directionality to the substrate incident sputtered particle flux. Directional emission refers to an angular distribution of as-emitted sputtered particles whose flux intensity is characterized by a distribution of particles in which the majority of emitted particle flux is contained within a narrow peak, or peaks, superimposed upon a low level background angular distribution. Ideally, the directionally emitted material arrives at the substrate at about the same one or few narrow ranges of angles most characteristic of emission from the target material. This makes it much easier to uniformly coat high aspect ratio features on the substrate.